1. Technical Field
An improved intravascular catheter is disclosed. More specifically, an improved catheter is disclosed that provides improved stress-strain behavior that exhibits a sufficient degree of stiffness at low cross-axial strains for sufficient pushability, and sufficient flexibility at higher cross-axial strains for sufficient trackability.
2. Description of the Related Art
Intravascular catheters are widely used for a variety of diagnostic and therapeutic purposes. Specifically, angioplasty has been developed as an alternative to bypass surgery for treating vascular diseases or other conditions that occlude or reduce blood flow in a patient's vascular system. Balloon angioplasty has proven to be a useful and often a preferred treatment for coronary diseases that cause blockages, also known as stenosis, in coronary arteries as well as other parts of the vascular system.
Advancing the catheter assembly to position the balloon across a stenosis can be a difficult and time consuming task due to the narrow and tortuous vascular passages through which the catheter assembly must be passed. The balloon must be positioned precisely and movement of the balloon through the vascular system must be conducted in as atraumatic manner as possible.
To be effective, the catheter assembly preferably has two distinct features. First, the catheter assembly must have sufficient “pushability” or axial strength thereby enabling a longitudinal force to be transmitted through the assembly so that the physician can push the catheter assembly through the vascular system to the stenosis. Concurrently, the catheter assembly preferably may also be sufficiently flexible so that the catheter assembly has good “trackability” so as to enable the physician to navigate the tortuous passages of the patient's vascular system.
To satisfy these criteria, balloon catheter assemblies typically have a stiff proximal end and a more flexible distal end. If a hypotube is used at the proximal section, it is typically manufactured from a metallic material, such as stainless steel. The balloon catheter or the distal section of the assembly is typically manufactured from a more flexible, polymer product. Thus, the hypotube is relatively stiff, enabling the assembly to have good pushability while the balloon catheter or tube is more flexible, enabling the assembly to have sufficient trackability.
One problem associated with connecting a relatively stiff tubular member, such as a hypotube, to a more flexible tubular member, such as the catheter, is that the transition between the stiff hypotube and the more flexible catheter can result in kinking which can close the lumen of the hypotube or the lumen of the catheter thereby blocking flow through these lumens to the balloon.
To solve this problem, stiffening members have been provided which help serve as a transition member between the hypotube and the catheter. Such stiffening members are disclosed in U.S. Pat. Nos. 5,658,251 and 6,066,114.
As angioplasty and stent delivery procedures continue to increase, there is a continuing need to provide new catheter systems and improved trackability and flexibility which can eliminate or reduce the need for hypotubes and stiffening members.
Returning to the competing interests of pushability and trackability, in general, the force (Fs) required to push a flexible shaft through a tortuous path is the sum of both the frictional forces (Ffr) and the force generated from bending the flexible shaft (Fb):Fs=Ffr+Fb  (1)where Fb is related to the flexural modulas of the shaft (Eb). Furthermore, it is known that when the force (Fs) on a given length of tubing (1) exceeds a certain critical value (Fsc), the tubing will buckle or collapse and thus limit both the trackability, force transmission of the catheter and fluid communication through the catheter:Fs>Fsc=π3/4Eb(Ro4−Ri4)/12→buckling  (2)wherein R1 and Ro are the inside and outside radii, respectively. As a result, a balance exists with regard to the trackability and pushability of a catheter as a function of the flexural modulus Eb. This balance is further illustrated in FIG. 1.
Referring to FIG. 1 at a low flexural modulus (Eb), the catheter will not have sufficient strength to overcome the frictional forces (Ffr) , Fs will exceed Fsc, and the tubing will buckle. On the other hand, at a high flexural modulus (Eb), the bending forces (Fb) within the tortuous path will be too large, Fs will exceed Fsc, and sections of the tubing will buckle. In addition, a high flexural modulus (Eb) can produce excessive counteractive force, producing guide catheter back-out.
Accordingly, a catheter material is needed that is relatively stiff at low bending strains, yet at larger bending strains is sufficiently flexible to minimize the bending forces.